Plug-ins are program modules that are installed as an extension of a host application to extend the functionalities of the host application. Examples of plug-ins include file format support for graphic software and media players, video players for Web browser, etc. The host application typically provides an open application programming interface (API) so that third-party vendors may provide plug-ins to interact with the host application.
Although plug-ins provide extension to the host program in a modular manner, their use has a number of disadvantages. First, the end user may not be sufficiently sophisticated to be able to select the appropriate plug-in to be used for a particular host application. Second, the plug-in has to be installed on the end user's computer, occupying storage space. Third, the end user has to get accustomed to the new features of the plug-in, often going through a long learning curve.